This invention relates to a wood screw.
As a wood screw driven into wood such as a wood board, one is known in which threads are formed on the outer periphery of a shank having a tapered portion at its tip, and the threads on the leading side, which are formed on the tapered portion at the tip of the shank, have a larger lead angle than the threads on the trailing side.
With such a conventional wood screw, since the leading threads have the function as drill edges, initial driving is smooth. But in the case of a wood screw having a long driving length, as the contact length of the threads increases, heat buildup increases, so that the threads tend to seize. In particular, if the wood into which the wood screw is driven is hard, much heat is produced in the threads, so that due to seizure, the wood screw tends to be broken.
In order to avoid such a problem, the present applicant has already proposed in JP utility model publication 5-14005 a wood screw in which two helical ribs of which the height gradually lowers from the longitudinal center toward both ends and which have an isosceles triangular sectional shape are formed on the outer periphery of the tip of a shank having threads on the outer periphery at intervals of 180xc2x0 in the circumferential direction such that the angle (lead angle) formed by the ridgeline of each helical rib and a plane perpendicular to the shank is greater than the lead angle of the threads.
In the wood screw described in this publication, when the wood screw is driven in, the ridgelines of the helical ribs penetrate into the wood while cutting off a threaded hole tapped by the threads on the tapered portion at the tip of the shank to form helical grooves, and the trailing threads in the rear of the helical grooves are driven into the inner periphery of the helical grooves. Thus the wood screw can be relatively smoothly driven into even hard wood. Heat buildup is small. Thus, it can be driven in without causing seizure.
In the wood screw proposed in the above publication, since the two helical ribs each have an isosceles triangular section and extend continuously in the longitudinal direction and the lead angle is greater than the lead angle of the threads, turning resistance imparted to the helical ribs is relatively large, so that a large force is needed to drive it in. Thus, improvement in lowering the driving force has been expected.
In order to reduce the turning resistance, it is effective to reduce the lead angle of the helical ribs or lower their maximum height. But by doing so, the groove width of the helical grooves formed by the helical ribs is decreased or their depth is reduced. This increases the resistance when the trailing threads are driven, so that heat buildup tends to occur.
An object of this invention is to provide a wood screw which can be smoothly driven into even hard wood with a small driving force.
According to this invention, there is provided a wood screw comprising a shank having a tapered portion at a tip thereof, threads formed on the outer periphery of the shank, two helical ribs formed on the outer periphery of the tip of the shank spaced 180xc2x0 from each other in the circumferential direction, the helical ribs being the highest at the longitudinal center and gradually lowering toward both ends, the maximum height of each helical rib being less than the height of the threads, the angle formed by the helical ribs and a plane perpendicular to the shank being greater than the lead angle of the threads, each of the helical ribs comprising a discontinuous body formed by a plurality of conical protrusions having different heights.
The discontinuous bodies as used herein refer to ones in which the apexes of the plurality of protrusions are completely separate from each other. But the bottoms of the protrusions may be separate or connected together.
By forming each helical rib from the discontinuous bodies of the plurality of conical protrusions having different heights, when the wood screw is driven in, it is possible to lower the turning resistance imparted to the helical ribs, so that the screw can be smoothly driven into even hard wood with a small driving force.
According to this invention, there is also provided a wood screw comprising a shank having a tapered portion at a tip thereof, threads formed on the outer periphery of the shank, two helical ribs formed on the outer periphery of the tip of the shank spaced 180xc2x0 from each other in the circumferential direction, the helical ribs being the highest at the longitudinal center and gradually lowering toward both ends, the maximum height of each helical rib being less than the height of the threads, the angle formed by the helical ribs and a plane perpendicular to the shank being greater than the lead angle of the threads, each of the helical ribs comprising a conical protrusion disposed at the center of its length, the height of the conical protrusion being smaller than the height of the threads.
By providing a conical protrusion at the longitudinal center of each helical rib as described above, it is possible to suppress the maximum height of the helical ribs. Thus it is possible to reduce the turning resistance imparted to the helical ribs. Thus, the screw can be smoothly driven into even hard wood with a small driving force.
In either of the first and second inventions, the protrusions may be conical or pyramidal.